Hey Nate,
I do know one thing, (I think) . The vin # on the 1972 identified the motor. A "w" indicated a 454 and a "U" indicated a 402 motor, etc. It did not tell us if it was a Custom or not. I do agree from what I have seen and been told, that on Canadian built Monte"s, A Z20 on the cowl tag for a SS454 and a Z03 for a 1972 Custom. I remember reading the post that Alex is referring to about Leo's Canadian built 71 SS454. Now if I could only remember what was said. Maybe Leo's is one that snuck out of the factory without it being stamped.
Vaughn